1. Field of the Invention
The present invention relates to composite metallic ultrafine particles and a process for producing the same, and more particularly to composite metallic ultrafine particles which have excellent dispersion stability and can be produced on an industrial scale, and a process for producing the same, and a method and an apparatus for forming an interconnection with use of the same.
2. Description of the Related Art
Generally, as a process for producing metallic ultrafine particles having a diameter of not more than 100 nm, there has been known a process in which metal is evaporated under vacuum in the presence of a small amount of gas, and then metallic ultrafine particles are condensed from the vapor phase to obtain ultrafine metallic particulates. However, this physical process suffers from practical problems that {circle around (1)} the variation in the particle diameter distribution is so large that even if a heating process is performed for film formation, grain boundaries are left, and a uniform metallic film cannot be obtained, {circle around (2)} the amount of metallic ultrafine particles produced in a single operation is so small that this process is not suitable for mass production, and {circle around (3)} devices for an electron beam, plasma, laser, induction heating, or the like are necessary for the evaporation of metal, and hence the production cost rises, for example.
Further, when the metallic ultrafine particles are solely taken out into the air, they are agglomerated. Therefore, it is necessary to disperse the particles in a solvent with use of a surface-active agent or the like. However, since the dispersion stability is poor, the storage stability is unsatisfactory.
There has been reported a method for producing metallic ultrafine particles in which metal ions produced from a metallic salt in an aqueous solvent are stabilized by a polymeric protective agent. However, such metallic ultrafine particles are limited to being handled in an aqueous system and thus have poor flexibility. Further, in this case, a high-molecular weight dispersant should be used for stabilization of metallic ultrafine particles. Therefore, metal content is lowered, and the ultrafine particles have little expectation of using as a metal source.
Metallic ultrafine particles have high activity and are unstable. Therefore, when such metallic ultrafine particles are gathered in a bare particle state, the particles are easily adhered to each other to cause an agglomeration or to be chained. In order to stabilize metallic ultrafine particles in such a state that the particles are separated from each other, it is necessary to form a certain protective coating on the surf ace of the metallic ultrafine particles. Further, in order that metallic ultrafine particles are stable as particles even with a high metal content, a metallic core should be stably bonded to the protective coating formed therearound.
When the production of metallic ultrafine particles on an industrial scale is taken into consideration, metallic ultrafine particles should preferably be safety and simply produced in a production process, and such metallic ultrafine particles are required to be utilized in various fields and to have good flexibility.
The present inventors have found the following thing. A certain metallic salt, a metallic oxide, or a metallic hydroxide is mixed with an organic compound including a functional group having chemisorption capability onto a metal produced from the metallic salt, the metallic oxide, or the metallic hydroxide. In a process in which the mixture is heated under the reflux condition of the organic compound for reaction, a core of an ultrafine particle of pure metal is produced by pyrolysis of the metallic salt, the metallic oxide, or the metallic hydroxide. The organic compound is chemisorbed onto the core by the functional group of the organic compound to form composite metallic ultrafine particles having a stable protective coating with high efficiency.
Further, it has been found that since a metallic salt, a metallic oxide, or a metallic hydroxide as a metal source and an organic compound for a protecting coating are different from each other, there is a possibility that the metal content, the reactivity, the particle diameter, or the like of the composite metallic ultrafine particles to be produced can be controlled by varying the combination of the metal source with the organic compound. Further, since the amount ratio of the metal source to the organic compound can also be manipulated as desired, processes from synthesis to purification of the composite metallic ultrafine particles can easily be optimized.
The present inventors have found that, in a process in which a certain metallic salt is mixed with an organic compound including an alcoholic hydroxyl group and pyrolyzed, an alcohol is bonded to the periphery of a core metal with a metal alkoxide bond to form composite metallic ultrafine particles having a stable protective coating. Further, the present inventors have invented a novel method for forming an interconnection on a semiconductor substrate with use of this composite metallic ultrafine particle, and an apparatus using this method.
The present invention has been made in view of the above circumstances. It is therefore an object of the present invention to provide composite metallic ultrafine particles which have more uniform particle diameters and have excellent dispersion stability and enhanced stability in the properties of particles, and a process for producing composite metallic ultrafine particles which can produce such composite metallic ultrafine particles simply and stably on an industrial scale.
According to a first embodiment of a composite metallic ultrafine particle of the present invention, there is provided a composite metallic ultrafine particle characterized in that a surface of a core metal produced from a metallic salt, a metallic oxide, or a metallic hydroxide and having a particle diameter of 1 to 100 nm is covered with an organic compound including a functional group having chemisorption capability onto the surface of the core metal.
It has been known that that the melting point of metallic particles lowers as the particle diameter decreases. This effect appears when the particle diameter is not more than 100 nm, and becomes significant when the particle diameter is 1 to 18 nm. When the particle diameter is 1 to 10 nm, some metals begin to melt even at around ordinary temperature. Therefore, the average particle diameter of the core metal (metallic ultrafine particle) is preferably 1 to 50 nm, and more preferably 1 to 18 nm, and, particularly, more preferably 1 to 10 nm. Further, the surface of the core metal is stably covered with the organic compound strongly chemisorbed onto the surface of the core metal with a chemical bonding strength. This organic compound serves as a protective coating for protecting the core metal, for thereby improving dispersion stability in a solvent and stability in the properties of particles. As the particle diameter of the core metal decreases, the proportion of the protective coating relatively increases to lower the metal content. Therefore, for some applications, it is not advisable to excessively reduce the particle diameter of the metal portion.
Here, chemisorption capability refers to formation of chemical bond only to the surface of the object without involving any further reaction. Thus, chemisorption capability is different from chemical reaction which implies reaction with the surface of the object so as to cut the bond between the surface atoms and the inside of the object material and to finally remove the surface atoms from the surface of the object.
Specifically, for example, in the case of copper, when a carboxylic acid is used as a protective group, as shown in the following scheme, the carboxylic acid is chemisorbed on the surface of copper at a low temperature, and, at a certain high temperature, a reaction occurs to disadvantageously remove surface atoms of copper and to form a copper salt of the carboxylic acid. This is considered attributable to the fact that, at such a high temperature, the carboxylic acid-copper bonding strength is stronger than the copper-copper bonding strength. Thus, in this case, the decomposition effect is superior to the protective effect, and, accordingly, when ultrafine particles of copper have been formed, even when copper ultrafine particles are formed, they are immediately decomposed to a copper salt of the carboxylic acid. 
On the other hand, for example, when a higher alcohol having five or more carbon atoms is used as a protective group, as shown in the following scheme, the alcohol is chemisorbed onto the surface of copper to form a monomolecular layer, which, even under considerably high temperature conditions, causes no more reaction and can serve as a protective layer, because the reactivity of the higher alcohol is not as high as that of the carboxylic acid. 
In the case of silver, both of the carboxylic acid and the higher alcohol have relatively low chemical reactivity compared with stability of silver. Hence, when the carboxylic acid and the higher alcohol are used as a protective group, these compounds are chemisorbed onto the surface of silver with no more reaction with silver. According to the present application, as described above, a combination of a metallic salt, a metallic oxide, or a metallic hydroxide as a metal source with an organic compound for the protective coating can be selected as desired. Therefore, as exemplified above, the protective coating can be optimized according to the properties of the selected metal.
As the core metal, there may be used at least one member selected from the group consisting of Ag, Au, Bi, Co, Cu, Cr. Fe, Ge, In, Ir, Ni, Os, Pd, Pt, Rh, Ru, Si, Sn, Ti, and V, for example. It is desirable that the amount of organic compound for covering the core metal is 0.01 to 1 molecule, per metal atom on the surface of the core metal.
As the organic compound, there may be used an alcoholic hydroxyl group, carboxyl, thiol, amino, or amide group which has four or more carbon atoms.
As the metallic salt, there may be used carbonate, nitrate, chloride, acetate, formate, citrate, oxalate, urate, phthalate, or a fatty acid salt having four or less carbon atoms.
According to a first embodiment of a process for producing composite metallic ultrafine particles of the present invention, there is provided a process for producing composite metallic ultrafine particles, characterized by comprising: mixing a metallic salt, a metallic oxide, or a metallic hydroxide with an organic compound including a functional group having chemisorption capability onto a surface of a core metal produced from the metallic salt, the metallic oxide, or the metallic hydroxide; and heating the mixture for reaction.
According to a second embodiment of a process for producing composite metallic ultrafine particles of the present invention, there is provided a process for producing composite metallic ultrafine particles, characterized by comprising: mixing a metallic salt, a metallic oxide, or a metallic hydroxide with an organic compound including a functional group having chemisorption capability onto a surface of a core metal produced from the metallic salt, the metallic oxide, or the metallic hydroxide; and heating the mixture under a reflux condition of the organic compound for reaction.
The above composite metallic ultrafine particles can be produced in a chemical process, and, therefore, can be mass-produced with use of a simple apparatus, without use of a large vacuum apparatus, in a usual atmospheric atmosphere. This contributes to lowered cost.
According to a second embodiment of a composite metallic ultrafine particle of the present invention, there is provided a composite metallic ultrafine particle having a structure in which a periphery of a core metal having a diameter of 1 to 100 nm is surrounded by an organic compound including an alcoholic hydroxyl group.
It has been known that that the melting point of metallic particles lowers as the particle diameter decreases. This effect appears when the particle diameter is not more than 100 nm, and becomes significant when the particle diameter is not more than 20 nm. The melting point is largely lowers when the particle diameter is not more than 10 nm. Therefore, from the viewpoint of use, the average particle diameter of the core metal (metallic ultrafine particle) is preferably 1 to 20 nm, and more preferably 5 to 15 nm. Further, the organic compound including an alcoholic hydroxyl group serves as a protective coating for protecting the core metal. It is advantageous, for example, in that, in use of the metallic ultrafine particles, the composite metallic ultrafine particles can easily be decomposed at a low temperature without significant obstruction by the protective coating. Furthermore, this can improve dispersion stability in a solvent and stability in the properties of particles.
The organic compound including an alcoholic hydroxyl group may be a straight-chain or branched-chain alcohol having four or more carbon atoms, or an aromatic compound including a hydroxyl group.
According to a third embodiment of a process for producing composite metallic ultrafine particles of the present invention, there is provided a process for producing composite metallic ultrafine particles, characterized by comprising heating an organic compound including an alcoholic hydroxyl group and a metallic salt as a metal source at a temperature that is not more than a decomposition initiation temperature of the organic compound including an alcoholic hydroxyl group and is not less than a decomposition temperature of the metallic salt.
According to a fourth embodiment of a process for producing composite metallic ultrafine particles of the present invention, there is provided a process for producing composite metallic ultrafine particles, characterized in that: a reducing agent, such as acetaldehyde, propionaldehyde, or ascorbic acid, in addition to an organic compound including an alcoholic hydroxyl group is added to a metallic salt as a metal source, and the mixture is heated to reduce the metallic salt.
The metal source may be at least one member selected from the group consisting of Cu, Ag, Au, In, Si, Ti, Ge, Sn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, V, Cr, and Bi.
The above composite metallic ultrafine particles can be produced in a chemical process in a liquid phase, and, therefore, can be mass-produced with use of a simple apparatus, without use of a large vacuum apparatus. This contributes to lowered cost. Further, since the composite metallic ultrafine particles can be produced at a low temperature, energy consumption can be reduced to contribute lowered cost. Furthermore, since the raw materials used are harmless to the environment, the composite metallic ultrafine particles can safely be produced on an industrial scale. Moreover, metallic ultrafine particles having a uniform diameter can be obtained in a nonaqueous system, and thus can be expected to be utilized in various fields.
According to a fifth embodiment of a process for producing composite metallic ultrafine particles of the present invention, there is provided a process for producing composite metallic ultrafine particles, characterized by comprising: dissolving or dispersing a metal source in a hydrophilic nonaqueous solvent to prepare a solution for composite metallic ultrafine particles; adding, to a hydrophobic nonaqueous solvent, an organic compound including a functional group having chemisorption capability onto a surface of a core metal produced from the metal source, and the solution for composite metallic ultrafine particles to prepare a precursor of ultrafine particles; and adding a reducing agent to reduce the precursor of ultrafine particles.
The above composite metallic ultrafine particles can be produced in a chemical process in a liquid phase, and, therefore, can be mass-produced with use of a simple apparatus, without use of a large vacuum apparatus, in a usual atmospheric atmosphere. This contributes to lowered cost. Further, since the raw materials used are harmless to the environment, loads on the environment can be small. Moreover, metallic ultrafine particles having a uniform diameter can be obtained in a nonaqueous system, and thus can be expected to be utilized in various fields.
An antioxidant may be added to the solution for composite metallic ultrafine particles to produce composite metallic ultrafine particles having enhanced stability. Even if a metal susceptible to oxidation is used, the addition of the antioxidant enables the composite metallic ultrafine particles to be synthesized. Further, the stability of the composite metallic ultrafine particles can also be increased, and it is possible to store the composite metallic ultratine particles for a long period of time. As the antioxidant, ascorbic acid or vitamin E may be used, for example.
The metal source may be at least one member selected from the group consisting of inorganic metallic salts and organometallic compounds (including complexes). It is desirable to use a metal source that is reduced at a temperature which is not more than the boiling point of the hydrophobic nonaqueous solvent in the presence of the reducing agent.
The metal constituting the metallic core may be at least one member selected from the group consisting of Cu, Ag, Au, In, Si, Ti, Ge, Sn, Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, V, Cr, and Bi.
Hydrophilic nonaqueous solvents may be, for example, alcohols having five or less carbon atoms, such as methanol and ethanol, and ketones such as acetone. Particularly, alcohols having five or less carbon atoms, such as methanol and ethanol are preferable. The amount (weight ratio) of hydrophilic nonaqueous solvent is preferably 1 to 40% with respect to the hydrophobic nonaqueous solvent.
The organic compound including a functional group having chemisorption capability onto a surface of the core metal may be at least one member selected from the group consisting of higher alcohols having six or more carbon atoms and surface-active agents, for example.
Citric acid or ascorbic acid may be used as the reducing agent, and the system may gradually be heated to a temperature at which the reduction action is developed.
The hydrophobic nonaqueous solvent may be at least one member selected from the group consisting of petroleum hydrocarbons, such as toluene and xylene, and terpenes, such as terpineol and turpentine. Particularly, petroleum hydrocarbons, such as toluene and xylene, are preferable.
As described above, composite metallic ultrafine particles according to the five embodiments, the melting point of the composite metallic ultrafine particles is extremely lowered, and a metallic film can be formed by a method similar to a type of baking finish. A novel method for forming an interconnection on a semiconductor substrate with use of this process has advantages with respect to an apparatus, the cost, and loads on the environment, unlike conventional vacuum evaporation, sputtering, and immersion plating.
A process according to the present invention basically comprises coating composite metallic ultrafine particles dispersed in a solvent on a semiconductor substrate on which a fine cavity for an interconnection is formed, drying and baking to form a metallic film, then polishing the surface of the substrate to form the interconnection, and cleaning and drying.
According to an embodiment of an apparatus for forming an interconnection of the present invention, there is provided an apparatus for forming an interconnection, characterized by comprising: a loading/unloading section having an inlet/outlet port; a dispersion liquid supply device for supplying a dispersion liquid of composite metallic ultrafine particles to a surface of a substrate, the dispersion liquid of composite metallic ultrafine particles being prepared by dispersing, in a predetermined solvent, the composite metallic ultrafine particles in which a surface of a core metal is covered with an organic compound including a functional group having chemisorption capability onto the surface of the core metal; a heating device for heating the substrate to melt the metal particles and bond them to each other; a polishing device for polishing the surface of the substrate to remove an excessively deposited metal, and a cleaning/drying device for cleaning and drying the polished substrate.
Preferably, the apparatus further comprises a supplementary drying device for drying a solvent contained in the dispersion liquid of composite metallic ultrafine particles which has been supplied to the surface of the substrate. The supplementary drying device can completely dry the solvent which has not been dried by simply spin-drying (air-drying) conducted in spin-coating, for example, so that formation of voids can be prevented during a heating process.
Preferably, the apparatus further comprises a bevel/backside cleaning device for cleaning a peripheral portion and/or a backside surface of the polished substrate.
Preferably, the apparatus further comprises a sensor for measuring a film thickness in at least one of times after evaporation of a solvent contained in the dispersion liquid of composite metallic ultrafine particles which has been supplied to the surface of the substrate, after a heating process in the heating device, and during or after a polishing process in the polishing device.
The sensor for measuring a film thickness may be provided in a substrate holding portion in a substrate transfer device for transferring a substrate. This can eliminate the needs for stop or interruption of processing the substrates and can increase throughput.
It is desirable that pressures in an indoor facility are respectively controlled in a cleaning division having the loading/unloading section and a cleaning/drying section housing the cleaning/drying device, and a treatment division having a dispersion liquid supply section having the dispersion liquid supply device therein, a heating section housing the heating device, and a polishing section housing the polishing device; and a pressure in the cleaning division is controlled so as to be higher than a pressure in the treatment division.
The treatment division in which chemical mist or gas due to chemicals used for each of the processes is dispersed, and the cleaning division for which a clean atmosphere is required are separated from each other so that the pressure in the cleaning division is controlled to be higher than that in the treatment division for preventing the air from flowing into the cleaning division from the treatment division. Hence, the chemical mist or the gas can be prevented from being attached to the substrate after formation of an interconnection.
According to an embodiment of a method for forming an interconnection of the present invention, there is provided a method for forming an interconnection, characterized by comprising: providing a substrate having a fine cavity formed on a surface of the substrate; supplying a dispersion liquid of composite metallic ultrafine particles to the surface of the substrate, the dispersion liquid of composite metallic ultrafine particles being prepared by dispersing, in a predetermined solvent, the composite metallic ultrafine particles in which a surface of a core metal is covered with an organic compound including a functional group having chemisorption capability onto the surface of the core metal; heating the substrate to melt the metal particles and bond them to each other; polishing the surface of the substrate to remove an excessively deposited metal; and cleaning and drying the polished substrate.